The ability of myosin to generate force and motion through its interaction with actin filaments is essential to many biological processes including muscle contraction, cell division, and intracellular transport. The atomic level structures of myosin in various stages of its enzymatic cycle have provided a framework of the molecular mechanism of force generation utilized by myosin. These structures as well as other biochemical and structural data suggest that myosin generates force by coupling small conformational changes in the nucleotide-binding region to a large swing of the light-chain binding region (lever arm) while myosin is strongly bound to actin. Mutations in human beta cardiac myosin are associated with several forms of cardiomyopathies, while it is unclear how the mutations lead to different disease pathologies. We propose the mutations alter the conserved structural mechanism of force generation by disrupting the subdomain coordination necessary for actin to activate the release of the products of ATP hydrolysis (phosphate and ADP) and trigger the force generating swing of the lever arm. We will investigate how the mutations impact specific conformational changes in the actin-binding, nucleotide-binding, and lever arm regions. Novel extrinsic fluorescence probes will be strategically placed to measure conformational changes in these three critical regions using fluorescence resonance energy transfer (FRET). In addition, transient kinetic experiments will be used to correlate the conformational changes with specific biochemical steps in the actomyosin ATPase cycle. The shift in the ensemble of structural states during key biochemical transitions will be examined by transient time resolved FRET. We will also investigate how the mutations alter the enzymatic and force generating properties of myosin, which will allow us to develop detailed models of how the mutations impair motor structure and function. We will determine how the cardiac myosin activator drug alters the conformational dynamics of human beta cardiac myosin and determine if it can rescue the altered motor structure-function in the cardiomyopathy mutants. Overall, our studies will be instrumental in developing therapeutic drugs that target myosin motor activity in heart failure and establishing the structural defects associated with cardiomyopathy mutations in myosin.